Transverse flow effect
Encyclopedia
Transverse flow effect is an aerodynamic effect encountered when a helicopter
moves (typically forward) through the air.
In a hover, the air above the rotor
disk is being pulled down from above and is equally distributed around the rotor disk. The air is descending from above, which has the effect of reducing angle of attack
.
As the helicopter starts moving into undisturbed air, a portion of the disk is in clean, unaccelerated air, while the remaining portion of the rotor disk is still working on descending air. The part of the disk working on clean air therefore sees a higher angle of attack than the portion of the disk which is working on descending air. This causes a difference in lift between the section in clean air and the section in descending air. The result is that the portion in clean air develops more lift, and the disk tilts as a result. Which way the disk tilts depends on a couple of factors:
Phase lag (rotorcraft)
will cause the extra lift to be seen approximately 90 degrees later in rotor rotation in semi-rigid two bladed rotor systems. Phase lag is a separate phenomenon from gyroscopic precession, which applies only to rigid systems. Rotor systems are not rigid systems since all helicopter rotors are designed to "flap" up or down as they change position around the rotor arc. This flapping counter acts dissymetry of lift in forward flight. Phase lag is a property of all rotating systems acted upon by a periodic force. For systems hinged at the axis of rotation ( in our case, a semi rigid flapping type rotor head) the phase lag is 90 degrees. For systems that are hinged at some distance from the axis of rotation (such as a fully articulated rotor head) the phase lag is less than 90 degrees. The pilot
will experience either a right or left roll (see flight dynamics
), depending upon whether the rotor of the helicopter rotates clockwise or counter-clockwise.
Which way the helicopter rotor disk is moving in the wind will determine which part of the disk has higher lift. For instance, hovering sideways, or hovering stationary in a crosswind, the clean portion of the disk might be on the left or right, rather than the front of the rotor disk. Again, depending on which direction the rotor rotates, this might be seen as a nose pitch up or nose pitch down, a roll, or something in between.
As the helicopter accelerates into a higher airspeed, more and more of the rotor disk will be in clean air and the lift differential will decrease. At some higher airspeed the effect will disappear. In a typical single rotor helicopter, the effect can be felt to start at around 5-10 knots, increases in magnitude to a maximum around 20 knots, and decreases above that until it is almost completely gone by 40-60 knots of airspeed.
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
moves (typically forward) through the air.
In a hover, the air above the rotor
Helicopter rotor
A helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight...
disk is being pulled down from above and is equally distributed around the rotor disk. The air is descending from above, which has the effect of reducing angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
.
As the helicopter starts moving into undisturbed air, a portion of the disk is in clean, unaccelerated air, while the remaining portion of the rotor disk is still working on descending air. The part of the disk working on clean air therefore sees a higher angle of attack than the portion of the disk which is working on descending air. This causes a difference in lift between the section in clean air and the section in descending air. The result is that the portion in clean air develops more lift, and the disk tilts as a result. Which way the disk tilts depends on a couple of factors:
Phase lag (rotorcraft)
Phase lag (rotorcraft)
Logically, it would seem that for a helicopter to roll to the left , lift would be required on the right and a downward force would be required on the left. That is, after all, how an airplane rolls to the left: ailerons on the trailing edge of the wings simultaneously increase lift on the right...
will cause the extra lift to be seen approximately 90 degrees later in rotor rotation in semi-rigid two bladed rotor systems. Phase lag is a separate phenomenon from gyroscopic precession, which applies only to rigid systems. Rotor systems are not rigid systems since all helicopter rotors are designed to "flap" up or down as they change position around the rotor arc. This flapping counter acts dissymetry of lift in forward flight. Phase lag is a property of all rotating systems acted upon by a periodic force. For systems hinged at the axis of rotation ( in our case, a semi rigid flapping type rotor head) the phase lag is 90 degrees. For systems that are hinged at some distance from the axis of rotation (such as a fully articulated rotor head) the phase lag is less than 90 degrees. The pilot
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
will experience either a right or left roll (see flight dynamics
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...
), depending upon whether the rotor of the helicopter rotates clockwise or counter-clockwise.
Which way the helicopter rotor disk is moving in the wind will determine which part of the disk has higher lift. For instance, hovering sideways, or hovering stationary in a crosswind, the clean portion of the disk might be on the left or right, rather than the front of the rotor disk. Again, depending on which direction the rotor rotates, this might be seen as a nose pitch up or nose pitch down, a roll, or something in between.
As the helicopter accelerates into a higher airspeed, more and more of the rotor disk will be in clean air and the lift differential will decrease. At some higher airspeed the effect will disappear. In a typical single rotor helicopter, the effect can be felt to start at around 5-10 knots, increases in magnitude to a maximum around 20 knots, and decreases above that until it is almost completely gone by 40-60 knots of airspeed.